Add support to Redis db index

I have verified this PR works with

[x] Online form submission
[x] Offline form submission
[x] Saving offline drafts
[x] Loading offline drafts
[x] Editing submissions
[x] Form preview
[ ] None of the above

What else has been done to verify that this works as intended?

Monitor Redis to ensure it does send "select <db index>" before sending the command

Why is this the best possible solution? Were any other approaches considered?

Maybe some of my PR could be merged with #1297 since the latter gives more leeway when using redis URLs.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

There should not be any regression risk because it falls back on db index 0 like it is in the current release. However, it lets the sysadmin to use 2 different databases when using only one instance of redis.

enketo / enketo

Add support to Redis db index #1298

I have verified this PR works with

What else has been done to verify that this works as intended?

Why is this the best possible solution? Were any other approaches considered?

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Do we need any specific form for testing your changes? If so, please attach one.