Email user when request to join instance approved

When a user's request to access an instance is approved they should
receive an email letting them know.

Committing for save

Update email conditionals

Committing for save

Committing for save

Rebased with master

Get notify mailer working

Delete unused files

Remove extra space

Update email text

Change notify back to devise mailer

Verify that email is working

Use letter opener to verify that email is sent and formatted correctly,
fix spec and static analysis errors

Remove unused functions

Re add removed test

Run bin rake gettext regen

Add negative test case

Add changelog entry

Remove username helper

Didn't need username helper, just used string interp

No plain strings

Use different string interp

Remove extra newlines

Add translation strings

Add equals sign

Address MR review comments

Clarify tests, DRY tests, add tests

Clarify email spec

Clarify deliver email to user email

Add missing space

Add html safe link

Clean up spec

Correct html safe link

Regex match link

Move link to email view

Move link out of email helper and into email view, which makes writing
specs way easier

Use percent interpolation instead of hash

Re run gettext regenerate

Change html haml email link

Rerun gettext regenerate

Match variable name

Rebase with master for merge conflict

Merge conflict resolution

Change resource to user

Fix static analysis errors

Change user back to resource