Don't use Genshi for HTML fragment creation

[Cinc-th]

Genshi template engine is used to create the HTML fragments inserted into the different Milestone pages. To start the transition to Trac 1.4+ fragments must be created in another way.

Note that this ticket is not about the usage of ITemplateStreamFilter.

Change

Remove Genshi templating when creating HTML fragments by

• removing MarkupTemplate() and MarkupTemplate().generate()
• rewriting templates used internally to use standard python templating

Don't use Jinja as a templating engine so we don't break the plugin for Trac 1.2.

[Cinc-th]

In 17777:

No longer use Genshi templates when creating HTML fragments. Add a few test cases for checking fragments.

Closes #13832

[Cinc-th]

This works with Trac 1.2 and Trac 1.4. For the latter Genshi must be installed.