Add titles for pages that lack them

[dcsjapan]

Some pages show nothing but the forum name in the title bar. The following titles can be added:

• "All Discussions" for the All Discussions page
• "Tags" for the Tags page
• The tag name when a tag-specific discussion list is displayed

In addition, a routine is needed to omit the title of whatever page is set as the site homepage.

[askvortsov1]

I've added titles for the tags pages in flarum/tags#72. I'll add the "All Discussions" title, and the logic for clearing the homepage in a new PR.

[askvortsov1]

What's better than 1 pull request? 6 Pull Requests! Here we go...

1. Added all discussions title to all discussions page, created mechanism to clear title if on defaultRoute
   Core: Add index page title, add mechanism to clear title from defaultRoute. #2047
   Lang pack: Add 'all discussions' meta title lang-english#158

2. Add title to 'all tags' and individual tag pages:
   Tags: Added title and description meta tags tags#72
   Lang pack: Add title and description to tags and tag pages lang-english#154

3. Add title to 'following' page:
   Subscriptions: Added meta title to following page subscriptions#23
   Lang Pack: Add meta title to following page lang-english#159

Have fun y'all!