Enforce Go style rules across codebase

Apply five style rules: convert iota string enums to typed
string constants, replace errors.As with errors.AsType,
merge three-group imports into two groups, fix multiline
parameter/argument formatting, and replace fmt.Sprintf URL
construction with net/url.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/iam/authorizer.go

@@ -101,12 +101,11 @@ func (a *Authorizer) authorize(ctx context.Context, tx pg.Tx, params AuthorizePa
 			*params.Session,
 			membership.ID,
 		); err != nil {
-			var (
-				errSessionNotFound *ErrSessionNotFound
-				errSessionExpired  *ErrSessionExpired
-			)
+			if _, ok := errors.AsType[*ErrSessionNotFound](err); ok {
+				return NewAssumptionRequiredError(params.Principal, membership.ID)
+			}
 
-			if errors.As(err, &errSessionNotFound) || errors.As(err, &errSessionExpired) {
+			if _, ok := errors.AsType[*ErrSessionExpired](err); ok {
 				return NewAssumptionRequiredError(params.Principal, membership.ID)
 			}