Enable Rust highlighting

[rfwatson]

.rs files are supported by highlight.js but weren't enabled in the backend.

Demo: https://git.rfwatson.net/rob/plume/src/commit/6a1572217b46e0cfd17222a5c5eabcaccbde329f/plume-models/src/password_reset_requests.rs

[codecov-io]

Codecov Report

Merging #7125 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7125      +/-   ##
==========================================
+ Coverage   41.56%   41.57%   +<.01%     
==========================================
  Files         446      446              
  Lines       60848    60848              
==========================================
+ Hits        25293    25297       +4     
+ Misses      32272    32267       -5     
- Partials     3283     3284       +1

Impacted Files	Coverage Δ
modules/highlight/highlight.go	31.81% <ø> (ø)	⬆️
models/repo_list.go	72.08% <0%> (-1.02%)	⬇️
routers/repo/view.go	43.03% <0%> (+1.01%)	⬆️
models/unit.go	67.56% <0%> (+5.4%)	⬆️

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