Add widget commit version to Materia

[iturgeon]

We need a migration to add widget_version (name up for debate) to widget table. This will be used to contain the git commit used to build the widget.

• [ ] add version to the admin widget interface
• [ ] Allow admins to edit the value on the admin page.
• [ ] cli widget installer looks for BUILD_GIT_COMMIT file in root of unzipped widget and use the value contained - note this file may not exist, needs to continue if it doesn't
• [ ] admin widget installer use the BUILD_GIT_COMMIT file if exists

[FrenjaminBanklin]

Also: widgets should probably keep track of original installation date and last update date separately.