Removing unused access_level parameter.

Bug introduced here -> 6d103a2f
access_level parameter wasn’t needed since -> 88c26393

This slightly changes the semantics of the 'New Issue' and 'New MR' events to
include new mentions in edited Mentionables.

An alternative would be to introduce 'Issue updated' and 'MR updated' events,
but that would lead to questions about why those events were only available
to new mentions, and not existing mentions as well, so hold off for now.

This is to ensure we don't send unwanted notifications when deleting a
project. In other words, stop abusing AR callbacks and use services.
Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

This reverts commit 13e37a3e.

+ Move 'Edit Project/Group' out of membership-related partial
+ Show the access request buttons only to logged-in users
+ Put the request access buttons out of in a more visible button
+ Improve the copy in the #remove_member_message helper
Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

[ci skip]

1. Make the "subscribed" text in Issuable sidebar reflect the labels
   subscription status

2. Current user mut be logged-in to toggle issue/MR/label subscription

1. Allow subscribing (the current user) to a label

- Refactor the `Subscription` coffeescript class
  - The main change is that it accepts a container, and conducts all
    DOM queries within its scope. We need this because the labels
    page has multiple instances of `Subscription` on the same page.

2. Creating an issue or MR with labels notifies users subscribed to those labels

- Label `has_many` subscribers through subscriptions.

3. Adding a label to an issue or MR notifies users subscribed to those labels

- This only applies to subscribers of the label that has just been
  added, not all labels for the issue.