Email text link colors should be separately configurable

[boonebgorges]

Currently, email text link color is set to highlight_color. Because of the weird dance of bp_email_add_link_color_to_template(), the only way to make the link color something other than highlight_color is to hook your own version of bp_email_add_link_color_to_template().

I suggest having link_color be a separate item that can be configured by filtering the 'email_appearance_settings' bp_parse_args() hooks. If link_color is not set explicitly, it can fall back on highlight_color.

[djpaul]

In 11891:

Emails: add support for changing email text link colour.

Adds a link_text_color option to bp_email_get_appearance_settings(),
which allows a developer to filter the email text link colour. This new
colour option is not exposed in the Customiser; that continues to use the
"highlight" colour for email text links.

Fixes #7646