Remove API and switch to api.fink-portal.org

[JulienPeloton]

Closes #689

Currently the code can be deployed locally as it completely relies on an external API :-)

Implemented so far

• remove all the API from the portal, and point to https://api.fink-portal.org. This includes the change of endpoints: /columns to /schema and /bayestar to /skymap
• use io.StringIO when decoding json in pandas (deprecation warning)
• remove all the microlensing section. (1) I never had any feedback in 5 years, (2) without a good baseline, Fink-only data makes no sense, (3) the version of pyLIMA pinned was 0.8.3 and it is completely obsolete. If anyone convinced me that it is useful to have it in Fink, I will be happy to resurrect it.
• remove a lot of unused code anymore
• update packages in requirements.txt to their latest (compatible) versions.

What remains

• keep cleaning the code
• update README with local installation for testing
• revamp the datatransfer service (rely on pyspark & fink-science which are big packages). Perhaps externalising it? This would have the advantage to not perturb the main API, and benefit from security.