Update datepicker to prevent choosing observation dates or running difference dates outside of observatory's data range

[dgarciabriseno]

This issue was created during a discussion about #372
We decided that the situation described in #372 should not be allowed during normal operation.
We should not allow users to select dates outside of the observatory's available date range.

This will be done in 2 steps:

1. Create an API endpoint that returns the start and end dates of data available for an observatory.
2. Query this API to get the bounds, then set the min/max date parameters on the datepickers.

[bogdanni]

getDataSources actually returns start & end dates. JHV displays them in tooltip when hovering over a dataset.

[dgarciabriseno]

Oh great! That saves me a lot of work. Thanks for pointing that out