Avoid preloading unused messages in ContribsPager

* newarticle
* rev-delundel
* rollbacklink

A bit of history:
* messages preloading
** added by Brion Vibber
   commit 935bca8ee764847868ce6c597b43eb74e0908c10

* usage of newarticle
** added by Erik Moeller
   commit d61d3486f0eb7713e58d80f984891cb8882cf7ef
** removed by Alexandre Emsenhuber
   commit 84be101a9098166731fe6a1e30615d2f9b094eeb

* usage of rev-delundel
** added by Aaron Schulz
   commit 0ed239362c54656b95d4d602fcc33d6f5ace8932
** fixed by Raimond Spekking
   commit 349ef821d66a54ee0c6b7ce6024bded399518e95
** removed by Aaron Schulz
   commit 90dcb3c1f2c4b03f2cb9358b40666af1eff88a99

* usage of rollbacklink
** present in Lee Daniel Crocker's 'initial revision'
   commit d82c14fb4fbac288b42ca5918b0a72f33ecb1e69
** removed from old version by Aryeh Gregor
   commit 3a0c45e93cf7cdcfadfd2545a497562968e023ee
** unused but still preloaded in Ilmari Karonen's new version
   commit 2df501fa0d2723bf2a196390c2507a6043cdcdaf

Change-Id: Ib9fce5c1af4f0fd7dd3dcfefa4a6611d61123e02