Feature/basic finder chart

[eheinrich]

Adds basic finder chart annotations to the aladin viewer on the target details page.

Additions include:

• Set the field of view
• Circle the target on the aladin image
• Compass drawn on the aladin image
• Scale bar of configurable size drawn on the aladin image
• Ability to save the resulting image

This only applies to sidereal targets. I also excluded the aladin viewer from being displayed for non-sidereal targets because it does not work with those target types.

Between a range of declinations from -85 to 85 degrees, within which most targets are expected to reside, the finder chart annotations look nice and clean. Outside of that range, as the target approaches a pole, they start getting more and more lopsided. This is a general issue when working close to the poles, and seeing as it probably will not affect most targets, I think it can be addressed at a later date to improve the annotations when close to the poles.

I made sure the style of the UI elements that were added match the look and feel of the rest of the target details page.

Here is a finder chart that was generated with this branch for m88, with a field of view of 10 arcmin.

[dmcollom]

This looks fantastic! The only thing I think we should do is briefly document the feature somewhere in the docs. Just updating the docstring for the Aladin templatetag to note the other features should be sufficient, but @rachel3834 may have other thoughts on that.

As a side note (and completely not your problem), the templatetag docs don't seem to be generating correctly in readthedocs, but they do locally for me. So if you make that change and don't see it reflected on RTD...don't worry about it, I guess.