Add ExecutiveMember model

[nishant-nayak]

Description

Adds the Executive Member model. Also includes small changes like:

• Scope the GENDERS list to the User model.
• Update the max length for phone number to 15 and add validator/pre-save sanitization of phone number.
• Copy the SIG model definition from Migrating pages #68 . Required for Executive Member dependency. SIG Pages will still be tracked in Migrating pages #68 .
• Small style fixes on login and index pages, mainly for better readability.
• Run pre commit hooks on all relevant files

Fixes #45

Dependencies

N/A

Type of Change

What types of changes does your code introduce?
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• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• 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. Please also list any relevant details for your test configuration. (for bug fix / breaking change)
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• Tested applying migrations onto existing changes on main. Didn't break any data.

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Checklist

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• My code follows the style guidelines of this project
• 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

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