Improve UI file naming/patterns

[ryanahamilton]

• Drops an unnecessary use of "master" from the codebase in the spirit of Consider renaming master branch to something less offensive #9351.
• Establishes naming pattern (depicted below) where asset names correlate with the template name that consumes them.

Files (sample) before:

static/
├─ css/
│  ├─ graph.css
│  ├─ main.css
├─ js/
│  ├─ base.js
│  ├─ graph.js
templates/
├─ airflow/
│  ├─ graph.html
│  ├─ master.html

Files (sample) after:

static/
├─ css/
│  ├─ graph.css
│  ├─ main.css
├─ js/
│  ├─ graph.js
│  ├─ main.js
templates/
├─ airflow/
│  ├─ graph.html
│  ├─ main.html

[ryw]

Great commit @ryanahamilton

[github-actions]

The PR should be OK to be merged with just subset of tests as it does not modify Core of Airflow. The committers might merge it or can add a label 'full tests needed' and re-run it to run all tests if they see it is needed!