chore: use variable for static.ietf.org references

[NGPixel]

Closes #5687

[codecov]

Codecov Report

Merging #5692 (33b05b7) into main (6178519) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5692      +/-   ##
==========================================
- Coverage   88.74%   88.74%   -0.01%     
==========================================
  Files         285      285              
  Lines       39845    39851       +6     
==========================================
+ Hits        35362    35367       +5     
- Misses       4483     4484       +1     

Impacted Files	Coverage Δ
ietf/meeting/forms.py	97.03% <ø> (ø)
ietf/doc/models.py	88.87% <100.00%> (+0.03%)	⬆️
ietf/meeting/models.py	86.35% <100.00%> (+0.04%)	⬆️
ietf/nomcom/utils.py	91.30% <100.00%> (ø)

... and 4 files with indirect coverage changes