Ds/small fixes improvements

[dsevillamartin]

Fixes #103
Fixes #69
Fixes #101

Changes proposed in this pull request:

• Use flexbox & grid layouts for UserDirectoryPage nav at the top
  • Improved search input to allow clicking anywhere & has focus styles
  • Improved mobile layout
• Add 'searchUsers' permission check & hide 'search all users' if not permitted
• Add option to link group mentions to user directory page
• Set title in UserDirectoryPage

Reviewers should focus on:
?

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).