gitweb: improve usability of projects search form

Refactor generating project search form into git_project_search_form().

Make text field wider and add on mouse over explanation (via "title"
attribute), add an option to use regular expressions, and replace
'Search:' label with [Search] button.

Also add "List all projects" link to make it easier to go back from search
result to list of all projects (note that an empty search term is
disallowed).

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>