Different informational text should be available for different roles

The application page contains a "slot" for informational text `:t.applications/intro`. While this is good, the instructions may need to be different for applicants and handlers. Especially in the situations where the handlers are not professionals (e.g. bona fide server). We could anyway have the option so that we can provide more useful instructions, or link to the manuals etc.

The logic can work so:
- if there is a text specified for the specific role (e.g. reviewer, applicant), then show that (could be `:t.application/intro-reviewer`)
- if there is a text specified for the general party (i.e., applicants/members vs. handler/reviewer decider) then show that (could be `:t.application/intro-applicants-and-members`)
- if there is a text for everyone then show that
(could be `:t.application/intro`)

We could show all the texts that match (should be in general just one). At least the default case should be backwards compatible (i.e., the one for all).