App Settings: Generated UI - single text input

[ErisDS]

Requires #2139

The goal of this issue is to be able to define a single text-based setting for an app as per #2139, and have this result in a working text input on that app's setting page (and not the setting page of any other apps).

The outputted UI needs to contain the label & help text defined in the app's settings, and a standard text input pre-populated with the value from the app_settings table.

When saving the setting, it needs to run any validations (such as max length) defined in the settings, and save the new value.

Notifications for success, validation error, or other errors need to be generated such that they are consistent with the rest of Ghost.

The key part of completing the goal of this issue is creating a structure for defining settings UIs that can easily be output, so that as we expand the abilities of apps and this system, it will be able to generate screens with many and various settings.

Data binding will be a serious consideration for this issue.

[ErisDS]

Temporarily closing all of the app specific issues (tagged with later). Will review & reopen those that are still relevant when we kick off that project, which should be shortly after Zelda lands.