Use flask Markup class

[Lukewh]

Found by @bartaz http://flask.pocoo.org/docs/1.0/api/#flask.Markup

[sowasred2012]

@bartaz do you have more info on this?

[carkod]

Is this required somewhere? is this an actual issue or is it just to share this tool?

[bartaz]

I think it was a suggestion on how to make it easier to use.

Currently image_template returns string, which has to be marked safe in the flask template (as mentioned in README):

{{ image( …. ) | safe }}

Using Markup class can let jinja template know that this is safe markup, so it can be inserted direcly without safe filter.

We use it in Vanilla as well:
https://github.com/canonical-web-and-design/vanilla-framework/blob/81202f14aa2f781d52344470de1092ff0ce24677/webapp/app.py#L177-L184