relay: code tweaks, README, and helper scripts for self-hosting

[bnewbold]

• Makefile command to build dashboard
• small refactors and simplifications of bigsky env var and CLI arg parsing; should be safe to merge and deploy to prod
• disable some Relay features by default when bigsky is run (like "aggregations" and "spidering")

The helper bash scripts are a bit duplicate with gosky commands, and aren't super important, I can yank those if they seem too redundant.

[ericvolp12]

One note on the signal trapping change but otherwise lgtm

[ericvolp12]

NVM Github rendered the change weirdly and made it seem like that was new code, it was just moved, LGTM.