add posibility to add image preview in show/edit page

[sulphur]

No description provided.

[aesmail]

Thank you @sulphur
This is definitely a good PR. I'd change some minor things though.

1. Instead of checking for the type inside the form_field function, you might want to move that inside the build_html_input function where all the types are already being checked and the function returns the necessary html.
   Check this line.

2. For the new type names, image_preview and custom_html are good, but we can make them shorter like image and html. Less typing, and meaning is preserved.

I think this PR is ready to be merged once this is done 👍🏼

[sulphur]

Hi, Sure I can change it to html and image. However I just wanted to point out before that html and image aren't form inputs that is why I didn't put them inside buitd_html_input since this function creates form elements. Tell me what you think and I will update the PR accordingly.

[artempankov]

is there any chance for merging this request? ))

[Faymir]

Hello, any news about this pull request ?

[aesmail]

I will try to merge this for the next release 👍🏻

[ghenry]

Hi @sulphur

Would you be so kind as to do this again to the master branch?

Thanks,
Gavin.