Add cookie for default signing profile

[yzernik]

Backend changes

Add two new routes to app.py to get and set the cookie:

• get_default_signing_profile_id
• set_default_signing_profile_id

Example of how to get and set cookie in flask: https://flask.palletsprojects.com/en/2.0.x/quickstart/#cookies

Frontend changes

Add two new functions to requests.js to request those endpoints:

• getDefaultSigningProfileIdRequest
• setDefaultSigningProfileIdRequest

Use getUserRequest as an example: https://github.com/yzernik/squeaknode/blob/f5b40054633b56b2713c3d8866f7193c6a0b9544/frontend/src/squeakclient/requests.js#L98-L104

Update MakeSqueakDialog.js component to get the profile id when component mounts, and set the profile id whenever the dropdown selection changes.

[yzernik]

This does not have to be a cookie. It can be a new field in the db "config" table: https://github.com/yzernik/squeaknode/blob/b0e438fc839b57d98421dd53f6c8f3c65fd49973/squeaknode/db/models.py#L184

[yzernik]

This can also be a field in the redux slice.