Improve text of MR merge widget and manual merge modal

Modal with manual merge instructions:

![Screen_Shot_2015-08-08_at_21.30.23](https://gitlab.com/gitlab-org/gitlab-ce/uploads/2bffe42e9ad76afd02efbe5816027767/Screen_Shot_2015-08-08_at_21.30.23.png)

Archived:

![archived](https://gitlab.com/gitlab-org/gitlab-ce/uploads/e1efa1d54ce7ead8de6e88ca34095c09/archived.png)

WIP:

![wip](https://gitlab.com/gitlab-org/gitlab-ce/uploads/14a831458d0946722559ec643f91678c/wip.png)

Missing target branch:

![missing_branch](https://gitlab.com/gitlab-org/gitlab-ce/uploads/340083bbacbc234906e58cf76c586a9e/missing_branch.png)

Missing source branch:

![missing_branch_source](https://gitlab.com/gitlab-org/gitlab-ce/uploads/72f80ed58efed88b2bbcf403b55b1a01/missing_branch_source.png)

Nothing to merge:

![nothing](https://gitlab.com/gitlab-org/gitlab-ce/uploads/85b0d9da87acacd53cffd0df58d8eed4/nothing.png)

Checking ability to merge:

![check](https://gitlab.com/gitlab-org/gitlab-ce/uploads/32d75322e622025610c446e8ca7d892d/check.png)

Conflicts when allowed to merge:

![conflicts](https://gitlab.com/gitlab-org/gitlab-ce/uploads/23fdb2c60a27d873b3cc8a008203eb59/conflicts.png)

Conflicts when not allowed to merge:

![conflicts_not_allowed](https://gitlab.com/gitlab-org/gitlab-ce/uploads/113462a80fc65bf48b20dc0efd808363/conflicts_not_allowed.png)

Mergeable when allowed to merge:

![accept](https://gitlab.com/gitlab-org/gitlab-ce/uploads/1b3bb1a71df21728fe9318e7a083bfba/accept.png)

Mergeable when not allowed to merge:

![not_allowed](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d38fa5c96f8f9d7223bb6bc7ad36f29d/not_allowed.png)

Merge in progress:

![locked](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b60dcf5eb70e6032b0e8aac766018df3/locked.png)

Merge failed:

![failed_2](https://gitlab.com/gitlab-org/gitlab-ce/uploads/3713b9a10298183935b028b658271ac9/failed_2.png)

Merged:

![merged](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7f68a48dd197a71351c9a80c59c8ecbe/merged.png)

Closed:

![closed](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d5ce040449b7db943bd2fbfffb607a10/closed.png)

cc @dzaporozhets @rspeicher

See merge request !1120