Validate the args of wordpoints_get_user_rank()

Formerly, on the rank group was validated, and then only if the user
wasn’t found in the user ranks database.

Includes unit tests.

Fixes #183
Merge branch 'issue-183'
Use number form input type where appropriate

See #181
Merge branch 'issue-181'
Ask for confirmation before deleting a rank

Fixes #184
Remove all empty form action attributes

They’re invalid in HTML5.

See #186
Don't have selected() print output directly

In these cases, we’re concatenating the output with the rest of the
HTML. Because `selected()` was `echo`ing the `selected` attribute,
invalid HTML was being produced.

Includes unit tests for the drop down builder.

See #186
Remove duplicate class attributes

The values weren’t duplicates, but these classes are unused and aren’t
necessary for targeting the elements via selectors.

See #186
Remove a class attribute from a hidden element

Found while working on #186.

Related: 945d2455352886cba5e53b38ce0beef2dfce92a7
Avoid dup. id's for delete points type buttons

See #186
Merge branch 'issue-186'
Update langage files

See #184
Merge branch 'issue-184'