In #23380 we want to change all ocurrences of `uri` to `url`. This
commit changes the ocurrences of `uri` appeared in files related to
email, including templates (`.html`, `.txt`) and backend (`.py`)
codes.
In `email.md`, `base_images_uri` is changed to `images_base_url` -
the words `base` and `images` are swapped and plural form is added
for `image`. This is becasue the former is not found anywhere in
the codebase while the later appears a lot. To reduce confusion,
this doccumentation changed accordingly.
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