front-end: Use select2 from node_modules and stop bundling it select2 3.5.4 was added in 304e83e9bcde ... but the latest npm release in the 3 series is 3.5.1, so we use that one instead. We should probably upgrade to the 4 series. The select2 images were not in the location the generated css pointed - now we copy them from node_modules to the right location, next to the generated css. Note: this will drop 190cb30841de "branches: fix performance of branch selectors with many branches - only show the first 200 results" ... but it should no longer be relevant now when we use server side filtering. 15e507047bae introduced select2-bootstrap.css - it is not clear what version was used, but we use the latest 1.4.6 which also is very old.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.