Make graphs more meaningful

[dArignac]

Currently we only see a fixed date range with all the prices of that range.
What is more meaningful is to only see when the prices changed as this it what matters if you consider buying or not.

[dArignac]

We could improve the whole workflow by only saving changed prices to database instead of every price on each API queue. This would clean the database enormously.
Don't forget a migration.

[dArignac]

If only the changed prices are saved, the changes of 55d8a07 can be reverted.