Add option to explicitly specify the use of STARTTLS

Required for servers that use STARTTLS without SMTPS such as Office 365

This uses a new 'ot' request parameter to indicate the object type of the content to return, which can be ither TAG or COMMIT.
If this is not provided, then COMMIT is assumed to maintain backwards compatability.
If tags are returned, then the paging parameters, 'l' and 'pg' are still supported, but searching options are currently ignored.

This change also ties the plugin manager into the Wicket framework and
allows plugins to contribute and mount new pages which are linked by the
top navbar and repository navbar extensions.

To allow for integration of 3rd party server monitoring solutions,
Gitblit needs to expose an extension point for collecting http data.