Add index information to database inventory

[AndrewVSutherland]

As noted in LMFDB/lmfdb#1403, we need to document and keep track of the indexes we are using, this has a big impact on the performance of the LMFDB, and lmfdb-inventory seems like the right place to do this. As an example/template, I have added index information to

https://github.com/LMFDB/lmfdb-inventory/blob/master/db-sato_tate_groups.md

(scroll down to the bottom to see the indexes).

[JohnCremona]

I have added index information to elliptic_curves (new) curves2 collection, still at https://github.com/JohnCremona/lmfdb-inventory/blob/master/db-elliptic_curves.md, but will expand it to match the Sato-Tate model. (All I did was paste in the output from [i['key'] for i in curves2.index_information().values()])
Later: the PR has this better formatted.

[JohnCremona]

The new inventory system viewable at http://beta.lmfdb.org/inventory/ does contain information about indexes (indices), for example click on "View Indices" for any of the collections at http://beta.lmfdb.org/inventory/abvar/

I will leave this issue open, since right now we have no easy way to edit this or update it. I will probably add an issue on the main repository for this.