BP messages search form lacks filter for markup

[hnla]

The directory search filters have filters on the html markup however the bp-messages-template.php bp_messages_search_form() hasn't.

Patch adds filter.

I've re-factored the approach slightly as not a fan of trying to read through mixed php/html strings and have pulled out some conditionals to new variables as easier to deal with.

Approach may not find favour :) this is a rushed task as it was a blocker to some work I was doing so needs checking over carefully.

[hnla]

Update message search for html filter

[johnjamesjacoby]

In 9349:

Improvements to bp_message_search_form():

• Escape gettext output in form field elements.
• Add filter to allow output to be manipulated.
• Bring surrounding code up to current coding standards.

Props hlna. Fixes #6113.