Prevent vote weight of 0

[MSoeb]

Description: Setting the vote weight in a meeting to value 0 is not possible. It can be changed to 0, but the value will not be saved. Also it is not possible to import a user in a meeting with the vote weight value set to 0.

Reproduction:

1. open a meeting
2. open Settings > Participants > activate option Activate vote weight
3. open Participants: create a new user, change the vote weight to value 0 and save it. -> value is not saved
4. open Participants > Three-Dot-Menu > Import: Import a new generated user with a vote weight set to 0 and check if it is correctly imported.

Note: Changing the value to 0 works, if the user can change data on organisation level in Accounts. If the default value of an account has changed to 0, it is possible inside a meeting to change the value to another number and back to 0 again.

What should happen:

• Accounts which can administrate meetings should be able to set the vote weight value to 0.
• user import: Import should work with the vote weight value set to 0.

[jsangmeister]

As discussed: vote weight of 0 should be forbidden, see OpenSlides/openslides-backend#1926. This should be consistently implement in all views.

[reiterl]

Prevent Zero in new-account dialog is done in #2911