[chore/frontend] Refactor header templating, add apple-touch-icon

[tsmethurst]

Description

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If this is a documentation change, please briefly describe what you've changed and why.

This pull request refactors our header template file to use nested templates for repeated values, tidies up some formatting for readability, and updates our use of 'icon' to drop 'shortcut' and add apple values (https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel#icon)

Closes #969 by making it no longer necessary.

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.

[f0x52]

looks good!