[wg/webapps] Rechartering Web Applications Group Charter (closing Services Worker WG)

[plehegar]

Evaluation

This would address 2 ongoing threads:

1. The work on Service Workers Working Group has stalled and it's difficult to maintain the current Working Group. The idea is to merge the work on Service Workers within the Web Applications Working Group
2. We also need to consider revising the success criteria section:
   [[
   In order to advance to Candidate Recommendation and to add features after reaching Candidate Recommendation, each feature is expected to be supported by at least two implementations, which may be judged by factors including existing implementations, expressions of interest, and lack of opposition.
   ]]
   from previous charter
   See also Add back old Success Criteria  webappswg#107

cc @siusin @ylafon @himorin @marcoscaceres @LJWatson

[plehegar]

(looks like item 2 is getting dropped)