Optimise searching for placeholders

If a string doesn’t have an opening parentheses then we can guarantee
it doesn’t have any placeholders in it.

We can take advantage of this to skip the expensive regex search.

Benchmark for string with no placeholders:
```shell
python -m timeit -s "import random; import string; from notifications_utils.field import Field; s = ''.join(random.choices(string.ascii_uppercase + string.digits + string.whitespace, k=1000))" "Field(s).placeholders"
```

Before
> 50000 loops, best of 5: 7.68 usec per loop

After
> 500000 loops, best of 5: 536 nsec per loop

14× faster

Benchmark for same string with one placeholder right at the end:
```shell
python -m timeit -s "import random; import string; from notifications_utils.field import Field; s = ''.join(random.choices(string.ascii_uppercase + string.digits + string.whitespace, k=1000)) + '((name))'" "Field(s).placeholders"
```

Before
> 50000 loops, best of 5: 9.06 usec per loop

After
> 50000 loops, best of 5: 9.06 usec per loop

No change.

Author: quis

notifications_utils/field.py

@@ -161,7 +161,7 @@ def formatted(self):
 
     @property
     def placeholders(self):
-        if not getattr(self, "content", ""):
+        if not self.content or "(" not in self.content:
             return InsensitiveSet()
         return InsensitiveSet(Placeholder(body).name for body in re.findall(self.placeholder_pattern, self.content))