Adding validation for the modals on the admin page

**As a** user **I want to** have a validation **so that** I can avoid common mistakes and the UI will be more user-friendly. Also there are key fields for creating source link for partner, but link is optional. So it seems like those fieds are required, but they aren't.

**Description**
Validation is a key for the correct interaction with the database (adding new info, editing the models, etc.) In addition, this is the way of avoiding bad requests from the database.

### Acceptance Criteria

- No bad requests after sending the forms.
- Valid data in the database.
- No empty fields in the tables.

**Epic link**
[Epic #2](https://github.com/ita-social-projects/StreetCode/issues/114)

### Tasks
1. - [x] Add validation to the all key fields of create partners modal.
2. - [x] Improve UI of create partners modal.
3. - [ ] Add validation to the all key fields of create news modal.
4. - [ ] Testing.