V2. Keep old bootstrap 3 view files included in separate folder, add mailViewsPath

[TonisOrmisson]

NB! this is MR to v2

Related to #488
We have not moved on in the plan for moving on with v2, I picked up this again, Bootstrap3 is not getting any fresher :)

Changes in this PR:

1. Include the Bootstrap 3 views in the base code for easier migration for BS3 users.
   Initially the plan was to have the old BS3 views in some separate repo, but it seems reasonable for easier transition to keep the old view files still included here.

I have split the views into two separate folders, one containing Bootstrap 5 (new) and Other containing Bootstrap 3 (old).
I have copied the BS3 files from the latest master branch.

2. The mail views are not dependent on an UI and it seems reasonable to me to keep the mail views separated and kind of part of the core. Also otherwise the BS3 and BS5 would both contain the same "mail". I have added a separate parameter 'mailViewPath' to the Module.

- resources:
     - views:
          - bootsrap3
          - bootsrap5
          - mail

Having the v1 views now in a different folder will create a problem for merges from master in view files, but it still kind of seems a clear way of distinguishing the versions.

3. Added the initial documentation to docs/guides/migrating-to-v2.md

Any thoughts whether this seems like a reasonable way to go forward or not are highly welcome.

NB! The view files of BS5 are not polished and need probably adjusting, but we need to get going on that path to go further.