Introduce indexes to db tables, greatly improving performance.

[dbrant]

This is a bit of a facepalm moment:
Our database tables of HistoryEntry and PageImage didn't have indexes set up! Adding indexes to tables (especially tables that will grow very large) is super important for performance when making queries or doing migrations.

I tested migrating a HistoryEntry table with 10000 entries. With our current db structure, the migration takes about 12 seconds, which accounts for the ANRs that we have seen from a small number of users.
With this new structure with indexing, the migration (with 10000 entries) takes 100 milliseconds.

Note that adding indexes will necessitate another db version bump (to 28), and I added two new migration paths, 27→28 and 26→28, simply because version 27 is already in the wild (in our Beta version). But really the 26→28 migration will be the one that occurs in production, and this is the one that is now optimized for efficiency.

[Williamrai]

I switched branch from the main to dbIndexes to test and then i got this this error view.
Image1

I did fresh install and added some history entry to have some data and then switched to this branch, then i saw this error view.
Image2

[dbrant]

I switched branch from the main to dbIndexes to test and then i got this this error view.
I did fresh install and added some history entry to have some data and then switched to this branch, then i saw this error view.

Whoops, absolutely right. I removed an unnecessary index creation statement from the 27→28 migration, which should fix this issue.

[cooltey]

Looks good to me! Will let @Williamrai to merge it if it looks good to you too.