shortening filter fix

[will-moore]

Fixes #194

Another python 2 > 3 fix. Use integer division to fix error from issue above.

To test:

• Check "Companion Files" tab of https://merge-ci.openmicroscopy.org/web/webclient/?show=image-83221 (user-3)
• Should be shortened with start...end (3 dots in the middle).
• Currently, the shortening fails (and is logged) but the full name is still displayed:

[manics]

LGTM.

You could optionally replace ... with a single unicode ellipsis … to allow more characters (or alternatively to add a space before/after), but that's not required.

[joshmoore]

I assume it would be good to roll this into a release, no? (I also wonder if there's any potential linting that would help us detect this type of error)

[will-moore]

It's not particularly urgent - I think it can wait for the next release, whenever that is.
For the user, it's a silent failure, and you just see the full name for companion files instead of the truncated one.

[will-moore]

I guess it would be nice to detect this kind of TypeErrors, but don't know if it's possible.
On the plus side, we don't tend to introduce these types of errors during development (as far as I'm aware!) - just still catching one's from Python 2 -> 3 migration.

[will-moore]

#203 attempts to catch remaining integer division errors.