Rename `Topic` to `Folder`

[bjester]

Desired behavior

To match UI strings in Kolibri 0.15, we should change the term Topic to Folder. This will require new translations.

Current behavior

Currently we refer to folder-like structures as topics.

[StarTrooper08]

Can I work on this issue?

[rtibbles]

Sure @StarTrooper08 - see the Getting Started guide in the README to get setup.

Note that this is only about changing user interface strings (these are generally found in Vue components in an object keyed under $trs, or in JS files with createTranslator). No need to change any non-user facing strings, as that's likely to break things!

[StarTrooper08]

ok, thanks.
I will have to look into it

[marcellamaki]

Fixed with #3307