Support builds that target a single wallscreen only

[thatbudakguy]

closes #213 branched from #235

this is a proof-of-concept demonstrating how we could create separate builds for each wallscreen. it relies on two important underlying changes to work:

• local media is separated by wallscreen instead of being in a single folder. this lets us exclude all the media files for wallscreens we aren't building. without it, we would be putting the entire 2GB silicon genesis video into every build, regardless of whether it was used.
• wallscreen templates do not explicitly include the HTML they use; instead they reference a layout file that inherits from the main wallscreen template. this avoids a path issue that occurs when limiting the scope of the build, and also lets us store additional metadata on the layout (e.g. a slideshow "knows" that its items are called slides and that it uses the slideshow stimulus controller).

if you check out the branch you'll get a lot of build warnings about files not being available — you can follow the warnings to move the contents of the local-media/ folder into each of the experience folders. each experience has its own local-media, thumbnails, attract-images, etc. all local-media folders are already gitignored. of course, you can also ignore the warnings and just test it using fallback images.

to test the build, you can invoke a build that includes a special config file, the example of which is _wallscreens/silicon-valley/_config.yml. this file will exclude other wallscreens from the build, and set the homepage to point at the wallscreen that's being built. to do so:

$ jekyll build --config=_config.yml,_wallscreens/silicon-valley/_config.yml

note that jekyll takes both config files in the order specified and merges them, so the latter one overwrites the settings of the former. the lack of spacing around the comma is important so that the shell doesn't misinterpret the command.

builds produced in this way will include only the media specific to the wallscreen being built, and upon starting a server (just substitute serve for build in the above command) will navigate to the "index" page for that wallscreen (see #235).

to create a build archive, you could use the above build command but append --destination [dir] which will output the build to the folder of your choice. you can then compress and upload it as an archive.

you could also use this approach as-is to generate one build per wallscreen; the result should be able to be deployed and work normally. you could add wallscreen-specific metadata to the per-wallscreen _config.yml files to make it available at build-time (for example, a unique analytics ID or environment value).

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[thatbudakguy]

closing pending later discussion on how to deposit wallscreen artifacts; see #196