PLT-1759 - Auto-complete for !channels when posting messages. (#3890)

* Auto-complete for !channels when posting messages.

This is part 1 of the fix for PLT-1759 to make channels linkable.

Still to do:
- Make the !channels clickable when they appear in messages. This is
  blocked until PR #3865 is resolved as it looks like that refactors
  some of the code that would be touched by making this change.

- Unit tests. Again, I think the above referenced PR should be merged
  before tackling this.

* Fix style problems.

* Highlighting of !channel-names in messages.

This only identifies the !channel-name (not the display name). The
implementation of the auto-complete on channel names now needs to be
modified to convert to the channel handle before sending the message.

* Display !channel-name as !Display Name.

When we encounter !channel-name in a message, display it as a link using
the channel's actual name rather than it's handle (name).

* Match on names and display name, and use name.

* Autocomplete channels matching on both the name and the the display
  name.
* Use the name as the text we fill in instead of the display name. It's
  potentially a bit ugly, but it minimises complexity for now as
  otherwise we'd have to do complicated things to the message box.

* Fix style issues.

* Load more channels everywhere.

Whenever we load the list of channels, we should also load the list of
more channels. This is to enable auto-completing and auto-linking of all
channels whether or not the user is in them currently.

* Include more channels in the map for linking.

* Listen for channel list updates for autolinking.

* Remove accidental console.log.

* Autocomplete on more channels too.

* i18n for channel autocomplete.

* Link directly to channels in !channel mentions.

This currently does not work if you aren't a member of that channel.
Need to decide what the correct behaviour is in that case.

* Fix style issues.

* Show channel name and handle in suggestion.

* Match channels only at start or after space.

* Better matching in text-formatting.

Only match channels after a space-type character or at the start in the
posts list too.

* Move the route construction to make tests work.

Moves route-construction out of text_formatting.jsx and into utils.jsx
so that the unit tests work once again.

webapp/utils/utils.jsx

@@ -1355,6 +1355,7 @@ export function handleFormattedTextClick(e) {
     const mentionAttribute = e.target.getAttributeNode('data-mention');
     const hashtagAttribute = e.target.getAttributeNode('data-hashtag');
     const linkAttribute = e.target.getAttributeNode('data-link');
+    const channelMentionAttribute = e.target.getAttributeNode('data-channel-mention');
 
     if (mentionAttribute) {
         e.preventDefault();
@@ -1372,5 +1373,7 @@ export function handleFormattedTextClick(e) {
 
             browserHistory.push(linkAttribute.value);
         }
+    } else if (channelMentionAttribute) {
+        browserHistory.push('/' + TeamStore.getCurrent().name + '/channels/' + channelMentionAttribute.value);
     }
 }