Move gitea URL config into board.yaml (pm-board PR #2 feedback)

The gitea_url top-level key in .pm/board.yaml replaces the .pm/gitea-url
marker file; pm-board and its task runner read config only from the
config file now (no backwards compatibility).

.pm/board.yaml

@@ -1,3 +1,4 @@
+gitea_url: https://git.jajahome.duckdns.org/omp/rsi
 requirements:
   - id: R1
     title: Jev research summary

.pm/gitea-url

@@ -1 +0,0 @@
-https://git.jajahome.duckdns.org/omp/rsi