Djblets 1.0 introduced SVG icons, which was used for higher DPIs. This

never made it into the resulting package, though, since these files were

left out of the manifest. While working to fix this, I noticed several

other files that were missing, such as *.map files and the

*-requirements.txt files.
This change carefully reworks our MANIFEST.in to better control what

files are included in the package. We now start by including the

entirety of the contrib, djblets, and docs directories, along with

some other specific files, and then selectively remove content from the

package. This gives us the various files like *.map and *.svg, while

keeping out files like *.py and vim swap files.
Some other files, like INSTALL and README, are now specifically

included. These were included before by default by setuptools, but

they've been added so that we can be more explicit about our coverage.
Files needed for building docs are also now present, allowing us to

build docs without needing a Git source tree.

Built packages before and after this change. Verified that the SVGs,

Map files, and requirements.txt files were included, and nothing was

missing.
Carefully inspected the file listings between what we've previously

shipped and now, and against the source tree.
Built all the docs from a source tree.