(#228) Cache unit rates and standing charges

[ryanw-mobile]

A basic implementation to cache Standard Unit Rates, Day Rates, Night Rates and Standing Charges in the Room DB.

Certain limitations are to be refined later:

• The cache will be considered as "hit" when the entire range of data exists in the DB, otherwise repository will try to download them again, even we have cached a portion of it
• We are still unclear when we a tariff has Direct Debit/Non-Direct-Debit or Unknown payment types. Right now we just pretty sure fixed and Agile should return "Unknown" while variable tariffs have DD/Non-DD. Future usage tickets have to look at this.
• We will need to migrate current standing charge data source from Tariff to the new repository standing charge method, which provides the true half-hourly data
• We do not have day/night rate tariff test data, and the app currently does not support such tariffs. We have the code here but we cannot unit test them yet.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +2.38% (target: -1.00%)	✅ 87.58%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (23f4d37)	1978	1269	64.16%
Head commit (0e9fb65)	2086 (+108)	1388 (+119)	66.54% (+2.38%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#237)	161	141	87.58%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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