Feedback from JOSS review

[stsievert]

What does this PR implement? It implements some changes in the JOSS paper review at https://github.com/openjournals/joss-reviews/issues/4517

Frontend:

• [x] allows some HTML/JS customization (and provides examples)
• [x] Has better documentation around frontend customization and deploying. Shows examples, and mentions on tips 'n tricks I've learned (at /frontend/)

Backend:

• [x] Allow offline to work with only minimal pip install

Documentation:

• [x] explain how crowdsourcing screens can be customized
• [x] offline install documentation improved.
• [x] clean up install docs (python fixed, prefer conda, etc)
• [x] Adds note about running Salmon in docker machine (only meant for devs).
• [x] check licenses more closely to make sure compatible (and adds note about licenses to docs)
• [x] hosts documentation on GitHub (as PDF and zipped HTML directory)
• [x] changes license to Apache BSD 3 (from MIT) (and adds note setup.py)
• [x] cleaned up the offline example code

Small improvements:

• [x] more stable version install with salmon.lock.yml
• [x] Removes starellete-prom (because it's has an incompatible license)

Reference issues/PRs

• Closes #137
• Closes #138
• Closes #135
• Closes #141
• Closes #140

[stsievert]

What does this PR perform?

• Makes documentation more accessible (PDF, HTML directory)
• Makes install more stable (pins versions, cleans up documentation for developers)
• Adds documentation page on customizing the frontend
• Changes license to BSD License
• Adds documentation page on deploying
• Adds dependencies and their licenses.
• Replaces starlette_prometheus with starlette_exporter
• Includes custom HTML for the top, middle and bottom, and custom JS too.
• Links to Salmon's license from the doc page

We have two copies of documentation on deploying. Resolved; now getting-started.html links to the deploying docs.

[stsievert]

Merging to update master. (of course will probably be more feedback from the JOSS review)