gitweb: Change the way "content tags" ('ctags') are handled

gitweb: Change the way "content tags" ('ctags') are handled

The major change is removing the ability to edit content tags (ctags)
in a web browser.

The interface was created by gitweb, while actual editing of tags was
to be done by external script; the API was not defined, and neither
was provided example implementation.  Such split is also a bit fragile
- interface and implementation have to be kept in sync.  Gitweb
provided only ability to add tags; you could not edit tags nor delete
them.

Format of ctags is now described in the comment above git_get_project_ctags
subroutine.  Gitweb now is more robust with respect to original ctags
format; it also accepts two new formats: $GIT_DIR/ctags file, with one
content tag per line, and multi-value `gitweb.ctag' config variable.

Gathering all ctags of all project is now put in git_gather_all_ctags
subroutine, making git_project_list_body more clear.

git_populate_project_tagcloud subroutine now generates data used for
tag cloud, including generation of ctag link, also in the case
HTML::TagCloud module is unavailable.  Links are now generated using
href() subroutine - this is more robust, as ctags might contain '?',
';' and '=' special characters that need to be escaped in query param.
Shown tags are HTML-escaped.

The generation of tag cloud in git_show_project_tagcloud in the case
when HTML::TagCloud is not available is now changed slightly.

The 'content tags' field on project summary page is made more in line
with other fields in "projects_list" table.  Because one cannot now
add new tags from web interface, this field is no longer displayed
when there are no content tags for given project.

Ctags-issue-Reported-by: Uwe Kleine-König <redacted>
Ctags-issue-Reported-by: Jonathan Nieder <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>