Narrow list of tags to related ones

When a tag is selected, the tag list narrows to show only the related
ones: all tags associated to the currently visible notes. Clicking on
the plus sign near another tag narrows the list again to the tags of
notes associated with the firstly AND secondly selected tag. To show
every tags again, press the tag icon on the top-left corner of
Boostnote.

Before:
![screencast](https://i.imgur.com/PwAdhLe.gif)

After:
![screencast](https://i.imgur.com/s3JCaFq.gif)

NOTE: Tags are joined with `&` character (`#` not works) in
`location.pathname` thus it will make the tags with this character
unavailable. Any suggestion to pass multiple values via pathname?

browser/main/SideNav/index.js

@@ -91,7 +91,7 @@ class SideNav extends React.Component {
     let component
 
     // TagsMode is not selected
-    if (!location.pathname.match('/tags') && !location.pathname.match('/alltags')) {
+    if (!location.pathname.match('/tags') && !location.pathname.match('/alltags') && !location.pathname.match('/narrowToTag')) {
       component = (
         <div>
           <SideNavFilter
@@ -145,10 +145,12 @@ class SideNav extends React.Component {
 
   tagListComponent () {
     const { data, location, config } = this.props
-    let tagList = _.sortBy(data.tagNoteMap.map(
-      (tag, name) => ({name, size: tag.size})),
-      ['name']
-    )
+    const relatedTags = this.getRelatedTags(this.getActiveTags(location.pathname), data.noteMap)
+    let tagList = _.sortBy(data.tagNoteMap.map((tag, name) => {
+      return { name, size: tag.size }
+    }), ['name']).filter(tag => {
+      return (relatedTags.size === 0) ? true : relatedTags.has(tag.name)
+    })
     if (config.sortTagsBy === 'COUNTER') {
       tagList = _.sortBy(tagList, item => (0 - item.size))
     }
@@ -158,6 +160,7 @@ class SideNav extends React.Component {
           <TagListItem
             name={tag.name}
             handleClickTagListItem={this.handleClickTagListItem.bind(this)}
+            handleClickNarrowToTag={this.handleClickNarrowToTag.bind(this)}
             isActive={this.getTagActive(location.pathname, tag.name)}
             key={tag.name}
             count={tag.size}
@@ -167,10 +170,33 @@ class SideNav extends React.Component {
     )
   }
 
+  getRelatedTags (activeTags, noteMap) {
+    const relatedNotes = noteMap.map(note => {
+      return {key: note.key, tags: note.tags}
+    }).filter((note) => {
+      return activeTags.every((tag) => note.tags.includes(tag))
+    })
+    let relatedTags = new Set()
+    relatedNotes.forEach(note => {
+      note.tags.map(tag => {
+        relatedTags.add(tag)
+      })
+    })
+    return relatedTags
+  }
+
   getTagActive (path, tag) {
+    const pathTag = this.getActiveTags(path)
+    return pathTag.includes(tag)
+  }
+
+  getActiveTags (path) {
     const pathSegments = path.split('/')
-    const pathTag = pathSegments[pathSegments.length - 1]
-    return pathTag === tag
+    const tags = pathSegments[pathSegments.length - 1]
+    if (tags === 'alltags') {
+      return []
+    }
+    return tags.split('&')
   }
 
   handleClickTagListItem (name) {
@@ -192,6 +218,20 @@ class SideNav extends React.Component {
     })
   }
 
+  handleClickNarrowToTag (name) {
+    const { router } = this.context
+    const { location } = this.props
+    let listOfTags = this.getActiveTags(location.pathname)
+    if (listOfTags.includes(name)) {
+      listOfTags = listOfTags.filter(function (currentTag) {
+        return name !== currentTag
+      })
+    } else {
+      listOfTags.push(name)
+    }
+    router.push(`/tags/${listOfTags.join('&')}`)
+  }
+
   emptyTrash (entries) {
     const { dispatch } = this.props
     const deletionPromises = entries.map((note) => {