webhtml: make top table header sticky

[bamarni]

This keeps the column names visible when scrolling down, which comes in
handy when there are many entries.

[codecov-io]

Codecov Report

Merging #518 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #518   +/-   ##
=======================================
  Coverage   67.06%   67.06%           
=======================================
  Files          37       37           
  Lines        7611     7613    +2     
=======================================
+ Hits         5104     5106    +2     
  Misses       2103     2103           
  Partials      404      404           

Impacted Files	Coverage Δ
internal/driver/webhtml.go	100.00% <100.00%> (ø)

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.06%. Comparing base (1ebb73c) to head (a86dd11).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #518   +/-   ##
=======================================
  Coverage   67.06%   67.06%           
=======================================
  Files          37       37           
  Lines        7611     7613    +2     
=======================================
+ Hits         5104     5106    +2     
  Misses       2103     2103           
  Partials      404      404           

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