Add pg as an optional dependency.

Closes #1333
- Change PostgreSQL build on Travis from an allowed failure to
  failing the tests just like sqlite3 and MySQL.
Remove image related text from meta description.

Closes #4408
Merge pull request #4395 from jaswilli/pg

Add pg as an optional dependency.
Use pure javascript library for postgresql.
Merge pull request #4410 from jaswilli/issue-4408

Remove image related text from meta description.
Change Numeric Keyboard to Keypad

Added pattern to html to make the keyboard a keypad

Closes #4426

    Added pattern="[0-9]*" to the postsPerPage input on general.hbs
Merge pull request #4422 from jaswilli/pg

Use pure javascript library for postgresql.
Merge pull request #4427 from dbalders/master

Change Numeric Keyboard to Keypad
Reset slugValue when the user changes

Closes #4411
- Previously, saving the user’s slug would lead to the slug value being
‘stuck’ on that saved value, even if one navigated to a different
user’s profile settings.
Merge pull request #4428 from felixrieseberg/iss4411

Reset slugValue when the user changes
full BREAD Tag endpoints and Tag api tests

close #4263
- implemented full BREAD Tag API
- deleting a tag will also detach its relations to posts
- tests of Tag API