Commit message textareas have 72 char mark line.

Fixes ACCEPTING MR at: http://feedback.gitlab.com/forums/176466-general/suggestions/5337507-show-72-character-mark-in-commits?tracking_code=284b0e5fd832f6885a68d90447c08d2a

All textareas that take a commit message now have the same monospace font in which they are shown under the `commits` tab, and a 72 character width vertical marker line.

I have only found commit message textareas in the following places:

- new, edit and remove file from web interface views.
- merge request custom commit message

Please notify me if there are any more of them.

**screenshots**

Edit file:

![screenshot from 2014-02-22 18 48 18 edit](https://f.cloud.github.com/assets/1429315/2238625/66b4990e-9bf8-11e3-94ec-2700c99f5678.png)

New file is analogous.

The remove popup had to be made wider because it did not have place for 72 characters before:

![screenshot from 2014-02-22 20 23 08 rm](https://f.cloud.github.com/assets/1429315/2238628/6e5a6120-9bf8-11e3-96bd-7035391823b0.png)

Custom MR commit message:

![screenshot from 2014-02-22 18 52 21 mr](https://f.cloud.github.com/assets/1429315/2238629/79d9a632-9bf8-11e3-8c72-daabfd0dba1c.png)

**doubts**

How to use exactly the same Bootstrap values of `textarea.form-control` `padding-left` for `.max-width-marker` `left`?

I could not find it, so I just set hardcoded the current Bootstrap value of `12px` for both the `padding` and the `left`.

This way it will never break, but it will not evolve automatically if the page style is modified.

One possibility to solve the two problems above: set all missing values ourselves in our SASS via variables to their current values so that those variables can be used in other places.