Stop using -[UIColor colorWithHexString:]

Replace `-[UIColor colorWithHexString:]` calls with `-[UIColor UIColorFromHex:]`.

The latter should be much faster and errorproof as it doesn't need to parse strings.

Fixes #2958
Merge pull request #2959 from wordpress-mobile/issue/2958-drop-colorWithHexString

Stop using -[UIColor colorWithHexString:]
Improves how a topics name for the url is obtained.
Merge pull request #2963 from wordpress-mobile/issues/2961-improve-remove-topics

Improves how a topics name for the url is obtained.
Merge branch 'release/4.7' into develop
Merge remote-tracking branch 'origin/develop' into issues/2945-comment-page-header-crash