feat: add brand_color variable for the email templates

[bydawen]

Added a color variable to simplify branding of email templates used on different platforms. Also added a variable for the maximum height of the email logo image.

The main purpose of adding these variables is to be able to control the color used on buttons, links, etc. in emails. To avoid making these changes in many files, it can now be done by changing the value of one variable in the base_body template.

For example, as in the screenshot below, the size of the logo, the color of the button and links affected by brand_color variable which is setted here with the logo height.
{% with brand_color="#1b511a" logo_max_height="65px" %}

Similar PR is opened to the master branch:
#33421

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[bydawen]

Hi! I'm contributing on behalf of Raccoongang and PR is ready for review, thanks!

[mphilbrick211]

Hi @bydawen! It looks like there's a failing check. Would you mind taking a look before we send to review? Thanks!

[bydawen]

Hi @bydawen! It looks like there's a failing check. Would you mind taking a look before we send to review? Thanks!

@mphilbrick211 hello, tests are fixed, ready to go on review, thanks!

[mariajgrimaldi]

We usually backport PR(s) after their corresponding master PR is merged. So we'll review this contribution against quince.master once that happens. Thanks!

[gabor-boros]

Hey @mariajgrimaldi 👋 What did you mean by

We usually backport PR(s) after their corresponding master PR is merged

I may misunderstand something, but the named master branches are never merged. (ref)

[cmltaWt0]

Mater PR is merged, but then reverted next day.
So I'm not going to merge this for now.

[mphilbrick211]

Hi @bydawen! Can this be closed, or does it need to remain open as a draft?

[mphilbrick211]

Closing for now due to inactivity.

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