Paginate + search for databases/tables on the homepage

[simonw]

Split out from #460 - in order to support large numbers of connected databases the homepage needs to be paginated.

[simonw]

I plan to add a search filter box too, but only if there are more than X (probably 10) connected databases.

[simonw]

This is needed by Datasette Library #417 since that's going to demand listing a LOT of databases on the homepage.

[simonw]

Related: redesign homepage entirely #991.

[simonw]

I've been thinking about this a bunch. For Datasette to be useful as a private repository of data (Datasette Library, #417) it's crucial that it can handle a much, much larger number of databases.

This makes me worry about how many connections (and open file handles) it makes sense to have open at one time.

I realize now that this is much less of a problem for private instances. Public instances on the internet could get traffic to any database at any time, so connections could easily get out of control. A private instance with only a few users could instead get away with only opening connections to databases in "active use".

This does however make it even more important for Datasette to maintain a cached set of metadata about the tables - which is also needed to power this feature.