Adjust view image and view exposure to allow for larger preview images

[bhilbert4]

Resolves #1584 (JWQL-214)

Based on feedback from the user survey.
Make preview images larger, to allow for easier QL checks.

[bhilbert4]

From testing using the current preview images, as well as the new preview images from #1592, the values to use for the image widths:
In view_image:
new preview images: 1000rem looks good. I tried switching to a percentage, but that moved the image to the right far enough that we couldn't take advantage of the smaller margin, and the image had to be smaller.
old preview images: Could go larger, but 1000rem looks fine.

view_exposure:
new preview images: 100% was best in all cases
old_preview images: With 2 detectors per row, 125% looked good. With 4 detectors per row, 110% looked good.

So it probably makes sense to keep the widths in view_exposure set to 100% (which is what they are set to already) under the assumption that eventually all programs will have the newer preview images created from #1592

For view_image, 1000rem seems reasonable, at least in terms of size.

Good test cases when working in the nircam test account:
http://127.0.0.1:8000/NIRCam/exposure/jw01062004001_02101_00001/ - 2 det/row, new prev images
http://127.0.0.1:8000/NIRCam/exposure/jw01068004001_02101_00001/ - 2 det/row, old prev images
http://127.0.0.1:8000/NIRCam/exposure/jw01063001002_02101_00001/ - 4 det/row, new prev images
http://127.0.0.1:8000/NIRCam/exposure/jw01409034001_02102_00001/ - 2 det/row, new, short, wide prev images
http://127.0.0.1:8000/NIRISS/exposure/jw01538065001_02107_00004/ - single det, new, tall, narrow prev image

[bhilbert4]

@mfixstsci @BradleySappington I think this is ready for review. My CSS is weak, so I'm not sure if there is a better way to do what I've done here. I basically made a copy of the margin settings from the Dashboard page and applied them to the image level and exposure level pages.

[bhilbert4]

Note that in the case of a very narrow but tall preview image, these changes make the preview image quite large. But that actually makes it a lot easier to see what's going on in such a small subarray. Example case when running on the nircam development account: http://127.0.0.1:8000/NIRISS/jw01538065001_02107_00004_nis/