Cache last sign in date in User table

[varmar05]

We want to have user's last successful sign in date available in User table so it is quickly accessible by APIs. We update the value at user login. Also there is a fallback option to calculate login dates from LoginHistory and cache the result.

In DB migration we only add new column and leave data to be updated on demand.

[coveralls]

Pull Request Test Coverage Report for Build 17584746203

Details

• 20 of 21 (95.24%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 94.318%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server/mergin/auth/models.py	11	12	91.67%

Totals
Change from base Build 17550000582:	0.002%
Covered Lines:	7553
Relevant Lines:	8008

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💛 - Coveralls