Bug 12098: Refactor can_*_subscription in C4::Serials

Bug 12098: Refactor can_*_subscription in C4::Serials

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Tested on top of patches for 12048 and 12080.

Subscription search
- superlibrarian, IndyBranches on/off - always sees all subscriptions
- superserials, IndyBranches on/off - always sees all subscriptions
- no superserials, IndyBranches on - only sees own subscriptions
Note: Subscriptions without branches will only show, when all subscriptions
      are visible. In a future enh it might be good to enforce setting a
      branch, when IndyBranches is used.
- no superserials, IndyBranches off - always sees all subscriptions

Subscription editing
- superlibrarian, IndyBranches on/off - can edit all subscriptions
- superserials, IndyBranches on/off - can edit all subscriptions
- no superserials, IndyBranches on - can only edit own subscriptons and
  subscriptions without branch
  NOTE: it would make sense to also allow Edit > Edit as new (duplicate)
  here, so one can copy the subscription from another branch to modify
  it for the own branch.

Passes tests in t, xt and QA script, also newly provided unit tests.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>