Automatically add MPL licence via pre-commit hook

[stevejalim]

Description

To ensure all Bedrock files have the MPL in them, we can add a pre-commit hook to update any files which are being added or edited. This will help plug any gaps as we go.

Hat-tip to @robhudson for pointing out this very useful pre-commit hook: https://github.com/Lucas-C/pre-commit-hooks#insert-license

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Success Criteria

• Pre-commit hook installed and works for Python files
• Pre-commit hook installed and works for JS files
• Pre-commit hook installed and works for SCSS files
• Pre-commit hook installed and works for bash scripts
• Pre-commit hook installed and works for Jinja templates
• Pre-commit hook installed and works for Fluent files

[pmac]

Jinja template files as well, e.g.

bedrock/bedrock/mozorg/templates/mozorg/base.html

Lines 1 to 3 in 45dfd69

	{# This Source Code Form is subject to the terms of the Mozilla Public
	# License, v. 2.0. If a copy of the MPL was not distributed with this
	# file, You can obtain one at http://mozilla.org/MPL/2.0/. -#}