Add abbreviation for sensor names

[Makram95]

I have some places in my app where all of the current sensor names are much too long. Ideally we can add a name abbreviation field to the Sensor type, with a max String length of 3 or 4. See the "DOH" as an example.

[dbrgn]

I'd suggest adding a "shortname" field to the sensors table. Length could be limited to 4 characters.

[rnestler]

Do we want to define the shortname for every user? Wouldn't it be better if they can define the shortname themselves per sensor in the App?

[Makram95]

I would prefer to define it for the user, keeps everything consistent if they use multiple devices / OSs / Web.

[dbrgn]

Yeah, probably makes sense to define a default shortname. Apps could still offer an option to customize the shortname, if desired.

In case we want to make the field required in the API: A migration could use fullname[..4] as initial value, and then we can update the sensors in the live database manually.