Pull PullReqState into its own file

Extract PullReqState into its own file for code readability.

Because PullReqState shares some constants with main and server, extract
those constants into a `consts.py` file as well.

homu/consts.py

@@ -0,0 +1,38 @@
+import re
+from enum import Enum
+
+STATUS_TO_PRIORITY = {
+    'success': 0,
+    'pending': 1,
+    'approved': 2,
+    '': 3,
+    'error': 4,
+    'failure': 5,
+}
+
+INTERRUPTED_BY_HOMU_FMT = 'Interrupted by Homu ({})'
+INTERRUPTED_BY_HOMU_RE = re.compile(r'Interrupted by Homu \((.+?)\)')
+DEFAULT_TEST_TIMEOUT = 3600 * 10
+
+WORDS_TO_ROLLUP = {
+    'rollup-': 0,
+    'rollup': 1,
+    'rollup=maybe': 0,
+    'rollup=never': -1,
+    'rollup=always': 1,
+}
+
+
+class LabelEvent(Enum):
+    APPROVED = 'approved'
+    REJECTED = 'rejected'
+    CONFLICT = 'conflict'
+    SUCCEED = 'succeed'
+    FAILED = 'failed'
+    TRY = 'try'
+    TRY_SUCCEED = 'try_succeed'
+    TRY_FAILED = 'try_failed'
+    EXEMPTED = 'exempted'
+    TIMED_OUT = 'timed_out'
+    INTERRUPTED = 'interrupted'
+    PUSHED = 'pushed'