Search only tickets by default

[johnbillion]

When performing a search on Trac, both tickets and changesets are included by default. This adds significant noise to the search results, especially for less experienced contributors who may not understand or even notice the difference between [12345] and #12345 at the beginning of each item's summary.

By default, only tickets should be searched when using the search form at the top of the page.

[netweb]

Looking at ​https://trac.edgewall.org/wiki/TracIni#search-section, this 3034.patch​ should take care of that, default search will search only tickets, disables changeset, milestone, and wiki across all w.org Trac instances.

[obenland]

In 7301:

Trac: Don't search changesets by default.

Props netweb, johnbillion.
Fixes #3034.