Rename database from `svs.sqlite` to `securedrop.sqlite`

[rmol]

Now that we've renamed the client VM, we should probably name the database something other than svs.sqlite, e.g. app.sqlite, securedrop.sqlite, sd.sqlite. Other suggestions welcome.

[eloquence]

securedrop.sqlite seems simple enough. Since it's internal, I'm OK with deferring it to later, but can also take a stab at it now if that'll make it easier to roll out.

[eloquence]

From 2022-08-22 review with @nathandyer @cfm @gonzalo-bulnes @eloquence:

• Worth revisiting next time we do a major DB migration/squashing of migrations. Keeping open.

[eloquence]

It's safe to say we won't use this name as part of the rewrite, and it's not worth renaming it for the old codebase at this point. Closing.