Ensure that the base_extrinsic weight does not change for Capacity tr…

…ansactions. (#1455)

# Goal
Ensure that the `base_extrinsic` weight does not change for Capacity
transactions.

Closes #1215

# Discussion
Created a new constant `CAPACITY_EXTRINSIC_BASE_WEIGHT` as a snapshot of
the benchmarks in: `runtime/common/src/weights/extrinsic_weights.rs`.

This change required updates to the unit tests, as this new constant
overrides the `base_extrinsic` in the `mock.rs`, the test accounts
required more tokens to cover the transaction costs.

# Checklist
- [ ] Chain spec updated
- [ ] Custom RPC OR Runtime API added/changed? Updated js/api-augment.
- [ ] Design doc(s) updated
- [ ] Tests added
- [ ] Benchmarks added
- [ ] Weights updated

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Co-authored-by: Matthew Orris <--help>