tigefa4u projects Trac

Small visual fix for label of Inter* links. When used as `[trac🎫1]` it did not show the prefix of 'trac', whereas `trac🎫1` (no brackets) did. Now they both do.

0.10.5dev: ported r6415 (''vc cache fix for postgresql 8.3'')

Adapt version control's cache to PostgreSQL 8.3. A few explicit casts of the revision to a str were missing.

Closes #6274.

Testing: merge through r6137, except r6120 and r6122. All tests pass.

Improvements to the `[[Image]]` macro:

* Absolute URLs now work fine in all cases. Closes #5704.
* Added support for server-relative linking: `//file`
* Added support for project-relative linking: `/file`

The last two work like the new 0.11 syntax for referencing server and project root.

Added a test case for each of the alternatives.

Testing: merge through r6128, except r6120 and r6122. All tests pass.

Testing: merge through r6123, except r6120 and r6122. All tests pass.

Testing: merge through r6121, except r6120, all tests pass

Testing: merge through r6119, all tests pass

Testing: merge through r6115, all tests pass