Refactor to Flask

[thomas-bc]

Refactoring from NodeJS+Express to Python+Flask.

There's likely more that could be done/implemented, although it seems to be functional so I'm opening for first round of review.
I changed the usage from needing an .fpv-env for locating the JSON files to using a CLI argument, defaulting to current working directory. There's also a CLI option for selecting the theme.

Review/Test recommendation

git clone https://github.com/thomas-bc/fprime-visual
git checkout refactor/flask
pip install .

fprime-visual --source-dir examples

cd examples && fprime-visual

fprime-visual --theme light-seafoam

Future Work

Publish to PyPi

[thomas-bc]

Adresses nasa/fprime#2060

[LeStarch]

Some things to think about. Also, long-term we should get away from a folders selection. The CLI should set the source folder (I think) and thus users should not need to set it.

[thomas-bc]

Introduced the following changes:

• screenshot button
• select-folder dropdown is hidden when only one folder is available