[MOON-1460] fix migration for author-slot-filter

[nbaztec]

This PR changes the following in author-slot-filter pallet:

• Fixes migration so the EligibilityValue is written even if old value was missing from storage (previously it would have ignored writing it).
• Adds try-runtime feature so the migration can be tested
• Adds default() method (for no-std in runtime) instead of name constant
• Adds migration tests

[notlesh]

In what cases is this value expected to be relevant? If it's mostly supposed to be a safe number, I'd set it to 100%. Nevermind, I forgot the whole point of this was to move away from using a percentage :)

[librelois]

It's more readable here to use an if let syntax

[nbaztec]

Personally, I prefer to avoid having to specify defaults in multiple places, hence went with the functional approach, but if the team prefers the procedural style, I can change it to.

               let new_value = if let Some(value) = old_value {
			let total_authors = <T as Config>::PotentialAuthors::get().len();
			let new_value = percent_of_num(value, total_authors as u32);
			NonZeroU32::new(new_value).unwrap_or(EligibilityValue::default())
		} else {
			EligibilityValue::default()
		};

Only disadvantage being a default specified in each branch. Alternatively we can remove the flatten (forgot about and_then), and specify default only once with:

               let new_value = old_value
			.and_then(|value| {
				let total_authors = <T as Config>::PotentialAuthors::get().len();
				let new_value = percent_of_num(value, total_authors as u32);
				NonZeroU32::new(new_value)
			})
			.unwrap_or(EligibilityValue::default());

thoughts?

[librelois]

If you prefer the functional approach, and_then is more readable here :)

[nbaztec]

I do prefer it but only due to not having to define a default in each branch (which would then need to be tested to not diverge).
In general I'd like to go with whatever the majority of the team is comfortable with :)

[notlesh]

I have always thought that the succinctness of if let some is lost when there is an else branch attached to it...

[librelois]

It's more readable here to use an if let syntax