No Validation of Edit Seating Position form

[mikerae]

Describe the bug
Rota Manager, on editng a player's position causes server error position is 0 or larger than allowed.

ref: #123

To Reproduce
Steps to reproduce the behaviour:

1. Log in as a rota manager
2. Click on 'edit player position'
3. allocate position 0
4. click on update player
5. See error

Expected behaviour
Validation only allows accepted input as per Add player Position:

• In accepted range 1 - max no of players needed for project
• not already allocated

Screenshot

[mikerae]

[#154] BugFix No Validation of Edit Seating Plan

1/ Cosmetic change: change field id to pk in line 284
2/ Add "position": seating_position, to context in line 308
- the delete button required a position.id, but was not being passed
one in the context.
modified: string_rota/views.py