Bug 989137 - Part 6: Create experiment XPIs during the build; r=bsmedberg

Instead of checking in binary .xpi files, we now have the source files
under source control and generate the .xpi as part of the build.

The build system mechanism to generate the .xpi files is hacky. But it's
similar to what existing processes use. Bug 988938 will eventually make
this more sane.

Since the produced .xpi files no longer have stable content, the SHA-1s
of the experiments are now calculated at test run time.

--HG--
extra : rebase_source : 85ae5a6c480a5add173b639f06b3ee404c5c6686
Bug 989137 - Part 7: Remove unncessary content from test experiments; r=Unfocused

The test experiments had unnecessary content in their install.rdf files.

<targetApplication> does not apply to experiment types and has been
removed.

The bootstrap setting is also implied and has been removed.

--HG--
extra : rebase_source : 52c297b17244b2cf160bc41018897ecc3fb0eabd