import: Fix handling of legacy and wildcard mentions.
import: Fix handling of legacy and wildcard mentions.

Our recently-added code for rewriting user IDs on data import didn't
correctly handle wildcard mentions and mentions generated by very old
versions of Zulip (pre data-user-id).

hackerkid/zulip,tommyip/zulip,brainwane/zulip,rht/zulip,hackerkid/zulip,rishig/zulip,tommyip/zulip,eeshangarg/zulip,synicalsyntax/zulip,brainwane/zulip,showell/zulip,eeshangarg/zulip,shubhamdhama/zulip,brainwane/zulip,punchagan/zulip,timabbott/zulip,andersk/zulip,zulip/zulip,rishig/zulip,tommyip/zulip,zulip/zulip,andersk/zulip,rishig/zulip,showell/zulip,andersk/zulip,synicalsyntax/zulip,andersk/zulip,eeshangarg/zulip,synicalsyntax/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,timabbott/zulip,rishig/zulip,kou/zulip,rishig/zulip,kou/zulip,timabbott/zulip,zulip/zulip,rht/zulip,timabbott/zulip,brainwane/zulip,punchagan/zulip,shubhamdhama/zulip,zulip/zulip,rht/zulip,tommyip/zulip,kou/zulip,showell/zulip,zulip/zulip,hackerkid/zulip,tommyip/zulip,hackerkid/zulip,shubhamdhama/zulip,rht/zulip,rht/zulip,synicalsyntax/zulip,kou/zulip,brainwane/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,shubhamdhama/zulip,hackerkid/zulip,andersk/zulip,showell/zulip,timabbott/zulip,shubhamdhama/zulip,zulip/zulip,eeshangarg/zulip,tommyip/zulip,kou/zulip,showell/zulip,timabbott/zulip,punchagan/zulip,showell/zulip,rishig/zulip,eeshangarg/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,andersk/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,rht/zulip,hackerkid/zulip,rishig/zulip,brainwane/zulip,shubhamdhama/zulip,synicalsyntax/zulip,kou/zulip,synicalsyntax/zulip,tommyip/zulip,brainwane/zulip,eeshangarg/zulip