templates: improve support for HTML escaping

JIRA 6.0 changes the default in Velocity templates from disabled HTML escaping to enabled HTML escaping.
One of the ways to avoid HTML escaping is to have methods whose name ends in "Html".
This change uses this technique to avoid unintended escaping of our links and linked commit messages.

https://developer.atlassian.com/display/JIRADEV/Velocity+Templates
templates: normalize HTML escaping

In JIRA 6.0 changes the default in Velocity templates from disabled HTML escaping to enabled HTML escaping.
To ensure the same behavior in both 6.0 and 5.1, it's recommended that the desired handling be specified in each template.

https://developer.atlassian.com/display/JIRADEV/Velocity+Templates
admin: fix "Git Repositories" link

The DefaultSimpleLinkManager had been encountering NullPointerExceptions trying to create the link in the sidebar.
When it encounters an exception, it logs it, and then ignores the link.
It looks like perhaps the section we were adding to doesn't exist any more; I changed to the section that the SVN plugin was using.
Merge pull request #9 from davidmc24/jira-6.x-support

Jira 6.x support