convert formhub labels to commcare labels

this converts any <input readonly="true()"> to a <trigger>.

This could be problematic if any manually-created forms or forms created
by another tool use an <input readonly="true()"> in a way that's not
compatible with just switching to a <trigger>, but seems unlikely that
this happens.
Merge pull request #111 from dimagi/formhub-labels

(MERGE AFTER #110) convert formhub labels to commcare labels