Prepare backend for being deployed as binary

[paulbricman]

The backend component should eventually be a self-contained binary with the following behavior:

1. It should first expose a limited API which can be used to get a snapshot.
2. After managing that, it should download and load the two auxiliary models used, the ones for fluid and descriptive search, respectively (#2, #5). The point is that while working behind the scenes to download those, the user can start using the alignment notebook.
3. The aligned model can be loaded if present, otherwise not. But this should change the behavior of the open dialogue function. It should essentially return more instructions, although those would have been presented initially, too.

[paulbricman]

Notes about how I debugged PyInstaller:

• Used this: https://github.com/pyinstaller/pyinstaller/issues/5428
• It was complaining about this zipimport, ran it with: pyinstaller server.py -D --additional-hooks-dir=hooks-files --hidden-import=zipimport
• Then, PyInstaller apparently doesn't include sacremoses: https://stackoverflow.com/questions/62080818/python-to-an-exe-file
• Then, transformers had to be tweaked and built from source: https://github.com/pyinstaller/pyinstaller/issues/5673