[FIX] website_mail.editor.js moved but still referenced

Commit 0c89ca0f8f2d96e37353348fe33591da8142368b moves
website_mail.editor.js but is still referenced in website_mail.xml
Merge pull request #263 from odoo-dev/master-fix-asset-fme

[FIX] website_mail.editor.js moved but still referenced

Commit 0c89ca0 moves
website_mail.editor.js but is still referenced in website_mail.xml
[FIX] website_mail*: fix + bundlify changes of 0c89ca: move subscription widget from website_mail to website_mail_group
[FIX] website_mail_group: restore missing snippet icon

The file was originally added in website_mail at 59b934
in saas-4 but was lost in master since.