2209 h5p exercise url

[TylerZeroMaster]

fixes openstax/ce#2209

Summary

• Add H5P exercise type (not part of resources because of the number of assumptions made about resources).
• Add note about paths being hardcoded into file system watchers and likelihood of watcher system changing.
• Check H5P exercise exists as part of page validation.
• Add H5P exercise url autocompletion.

Known issues

While making these changes, I found a few problems and I am working on making new issues for each since they are outside of the scope of 2209.

• Directory creation events are not handled at all. Moving a directory containing content that POET understands into an area POET is watching does not result in that content being added to the bundle.
• Page validation errors related to content not existing, i.e. resource or h5p, do not update until the page updates.
• Content paths are hardcoded into client where file system watchers are created.

[philschatz]

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