Introduce maxAwardedTotal achievement rule option

[cmfcmf]

This splits the maxAwarded option into two: maxAwarded and maxAwardedTotal, like discussed in #27. maxAwarded works as before, but maxAwardedTotal now looks at the total amount of the particular achievement ever granted to the user. This is achieved (hehe) by splitting the amount database field in the achievements table into current_amount and total_amount. current_amount is set to 0 when the achievement is replaced by another achievement, but total_amount only counts upwards. This makes replaces a bit more useful.

[cmfcmf]

@frederike-ramin could you please review again? We also need another reviewer, maybe @KBorchar?

[coveralls]

Pull Request Test Coverage Report for Build 92

• 22 of 22 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 97.094%

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Totals
Change from base Build 88:	0.5%
Covered Lines:	297
Relevant Lines:	301

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💛 - Coveralls

[cmfcmf]

This is ready for another review.