Define tox_conference_id_size and tox_conference_uid_size

[zugz]

Closes #1264

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This change is 

[iphydf]

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained

[codecov]

Codecov Report

Merging #1277 into master will decrease coverage by <.1%.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           master   #1277     +/-   ##
========================================
- Coverage    83.5%   83.4%   -0.1%     
========================================
  Files          82      82             
  Lines       14867   14829     -38     
========================================
- Hits        12420   12375     -45     
- Misses       2447    2454      +7

Impacted Files	Coverage Δ
toxcore/tox_api.c	61.2% <0%> (-2.7%)	⬇️
auto_tests/toxav_many_test.c	92.5% <0%> (-4.8%)	⬇️
toxav/audio.c	65.7% <0%> (-4.4%)	⬇️
toxav/msi.c	64.5% <0%> (-2.6%)	⬇️
auto_tests/toxav_basic_test.c	81.6% <0%> (-2%)	⬇️
toxcore/friend_connection.c	94.9% <0%> (-1.2%)	⬇️
toxav/toxav.c	67.6% <0%> (-1.2%)	⬇️
toxcore/LAN_discovery.c	83.9% <0%> (-1%)	⬇️
auto_tests/encryptsave_test.c	95% <0%> (-0.7%)	⬇️
toxcore/Messenger.c	86.9% <0%> (-0.1%)	⬇️
... and 5 more

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