i18n: Add regex for nested handlebars i18n.
i18n: Add regex for nested handlebars i18n.

This is required for our i18n search code to find the "Upload icon"
category of strings we're about to restore recently regressed i18n
for.

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