Making Serial Number the logic checked against for new imports

[akemidx]

Change to importing licenses.

When importing new licenses and the "update existing" is unchecked, the serial number will now be the unique identifier used for imports.

For licenses that have multiple seats on one serial number, users should enter the amount of seats in a column titled seats to match the importer syntax. Users can then assign seats out view the view page, or by clicking into the specific license and clicking on the seats tab.

NOTE: clicking the update existing will overwrite data if multiple matching serial numbers are in the import file. Thus, users are advised to be sure of their data before importing with update existing checked.

Fixes #
Freshdesk: 35948, 35412, et al
Github: #8947

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration
-Tested locally with sample imports of all combos of names/serial numbers

Test Configuration:

• PHP version: 8.0
• MySQL version
• Webserver version
• OS version

Checklist:

• I have read the Contributing documentation available here: https://snipe-it.readme.io/docs/contributing-overview
• I have formatted this PR according to the project guidelines: https://snipe-it.readme.io/docs/contributing-overview#pull-request-guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[what-the-diff]

PR Summary

• Improved License Importer
  The license importer now finds matching licenses using the serial number rather than the name, for better accuracy.
• Default Reassignable Setting
  If the reassignable attribute is not set, it will now default to '1' (true), allowing easier license management.

[uberbrady]

This looks great, and it's pretty impressive how few lines you were able to get it done in! I asked for one small change which should get us the "best of both worlds" - the old way, and the new way. If you can take a stab at making that fix and getting it tested (definitely check blank serials), then I'll feel pretty confident about getting it merged.