[IMP] Subscriptions: Renew subscriptions page rework

[fw-bot]

Big rework of the "Renew subscriptions" page

• Added appropriate icons to all smart button guilabels
• Removed unnecessary language from throughout
• Reorganized content into more logical sections (e.g., all "See Also" sections moved to end and merged)
• Reworked "Important" section at end with more accurate language, pending a rewrite of the "Scheduled actions" page
• Cleaned up the RST, ensuring guilabels are used correctly
• Cleaned up the RST, standardizing between bolding and italicizing for emphasis

This 18.0 PR can be FWP up to master.

Not sure if this reaches 50% revised, so I've marked it 2 points for now.

Forward-Port-Of: #14768

[robodoo]

[fw-bot]

This PR targets saas-18.3 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port