add node setting for welcome message #2675

#2675 

Todo:
* [x] restore success message ajax (currently breaks on vsite register form)
* [x] add behat test to verify submit ajax is working
* [x] get welcome overlay to work

**Todo (IN SEPARATE FUTURE PR)**
* Send admin notify email on site creation

#### Major Changes

##### 1. New welcome message setting

Vsite Register settings page has a new "Welcome message" setting. It's a node-autocomplete field that user 1 can set to be any node on the install. Visible at /admin/config/openscholar/vsite_register

![screeeeenshot](https://f.cloud.github.com/assets/228733/579745/5ff59ff4-c878-11e2-8211-921d3666a7a3.png)

##### 2. Welcome message behavior

When a new site is registered, the link that appears in the success message now links them to their front page, with the custom welcome message node as an overlay. This is done automatically via admin path site/register/welcome, which is only used in the link if the setting is specified as shown above.

![screen shot 2013-05-29 at 11 37 58 am](https://f.cloud.github.com/assets/228733/579641/4902e916-c876-11e2-8155-eade14703b2e.png)

#### Minor Changes

* AJAX success message restored (bug fix)
* AJAX success message style change (added button; see screenshot)

![screen shot 2013-05-29 at 11 32 00 am](https://f.cloud.github.com/assets/228733/579658/aa35b31c-c876-11e2-9b53-a0b5ffb0ea83.png)

* Site registration error fixed (related to Site visibility field save)
* Standardized user-facing text for Site visibility field.

New SIte visibility text on CP > Settings
![screen shot 2013-05-29 at 11 31 19 am](https://f.cloud.github.com/assets/228733/579668/c111c80a-c876-11e2-9c64-2676338a2d25.png)

New Site visibility text on Site Register
![screen shot 2013-05-29 at 11 31 32 am](https://f.cloud.github.com/assets/228733/579677/e199e2c4-c876-11e2-8646-70becc8ca844.png)

* Added verbose and label-only mode for field available values function

Label only-- displayed on node edit form
![screen shot 2013-05-29 at 11 30 49 am](https://f.cloud.github.com/assets/228733/579666/bafb117e-c876-11e2-8871-a4b3e29485ed.png)

* Removed development menu callback