Enable opening embedded images in image viewer lightbox

#### Is your suggestion related to a problem? Please describe.
The protocol supports messages like `"formatted_body": "text
",`, but contary to `"msgtype": "m.image"` these embedded images can't be opened and viewed in the lightbox at full resolution. In fact, everywhere an image can appear, it should be able to open like this (through several steps where needed, like user and room avatars).

#### Describe the solution you'd like.
same as m.image

#### Describe alternatives you've considered.
add an(other) official method to send an image description alongside an image, like
since it is often and easy that an image and it's description become disconnected.
probably that issue exists somewhere already since this is a good idea independed from this suggestion imo.

#### Additional context
see above