Upgrade Flask from 1 to 2

Flask is the backend framework. Due to several deprecation issues (e.g.: [a053f193969a7b2e3dde8ccf2b90b6a0092205f5](https://github.com/ubc/compair/commit/a053f193969a7b2e3dde8ccf2b90b6a0092205f5) ) requiring one-off workarounds, we should upgrade from Flask 1.1.x to Flask 2.x to get back onto the actively developed and maintained version.