Need to be able to run queries against multiple indices

[pgulley]

We need to be able to submit queries that search across all indices- the elasticsearch python api supports this already, but the Enum approach to listing the collections makes it impossible to actually run one from an external client. It would be as simple as adding a "mediacloud_searchtext*" to the collections enum here- although overhauling how we find the collections in the api might be a better long term solution.

[rahulbot]

99% of the time the end-user need is to be able to query against all the indexes at once, from both the API and the user interface this software provides. Open questions:

• Perhaps this can use the prefix that exists in the system already?
• Does this need an "alias" to be set up? First experiments suggest no.

It might be useful internally to keep a list of all the indexes that exist and make that available in addition #13.

[rahulbot]

Very related to #9

[pgulley]

addressed in #16