Create shard 3 for bok-choy; handle unknown shards.

This accomplishes three things:
1. Create a 3rd bok-choy shard
2. Allow the script to run 'all' shards
3. If the script receives an unknown shard, it will pass through empty tests.

The last one (3) is intended to allow us to expand to additional shards
on our Jenkins infrastructure. If we have, for example, a 4th shard introduced,
but someone attemps to run a commit that is older (and therefore still
has 3 shards); the script will just pass-through shard '4'. This means
older commits can still get tested. In that case, the older commit
will still test all cases, because the last shard (n) is basically an
'everything else' shard.
Split Bok-Choy Tests into 3 shards
TE-447
AWS settings for XBlocks to have filesystem-like storage
Merge pull request #4988 from edx/benp/all-tests-update

jenkins scripts: provide 'all' shard for bok-choy, use as default.
Merge pull request #5039 from edx/pmitros/aws-djpyfs

AWS settings for XBlocks to have filesystem-like storage
Merge pull request #5037 from edx/ammar-ben/bok-choy-shard-3

Split bok-choy tests into 3 shards (merge)
Revert "AWS settings for XBlocks to have filesystem-like storage"
Merge pull request #5040 from edx/revert-5039-pmitros/aws-djpyfs

Revert "AWS settings for XBlocks to have filesystem-like storage"
Add commands.
Work in progress.
Add fixes.
Fix keyboard navigation.
Add small fixes.
Fix styles.
Add fixes.
Add fixes.
Fixed typo and indentation.
Added positioning. Moved context menu code into its own module.
Collision detection works for menu. Sub-menu still has to be done.