Improve onboarding e-mail formatting

[joepavitt]

Description

Part of #3839

As part of the first steps of user-experience with FlowFuse, users are presented with plain text e-mails. Whilst functional, they do not look professional.

We use HubSpot in some cases to send nicely formatted e-mails, but should seek to provide nicer formatting/styling on these e-mails generated by the product.

Which customers would this be available to

All of them

Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

[joepavitt]

Considerations: https://flowfuse.com/handbook/design/branding/

E-Mails sent via HubSpot:

[joepavitt]

We should improve the format of the Team Invite e-mails whilst we're here too