Make package uploading work and add some docs about feature

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/user/project/maven_packages.md

+# GitLab Maven Packages repository
+
+## Configure project to use GitLab Maven Repository URL
+
+To download packages from GitLab, you need `repository` section in your `pom.xml`.
+
+```xml
+<repositories>
+  <repository>
+    <id>gitlab-maven</id>
+    <url>https://gitlab.com/api/v4/projects/PROJECT_ID/packages/maven</url>
+  </repository>
+</repositories>
+```
+
+To upload packages to GitLab, you need a `distributionManagement` section in your `pom.xml`.
+
+```xml
+<distributionManagement>
+  <snapshotRepository>
+    <id>gitlab-maven</id>
+    <url>https://gitlab.com/api/v4/projects/PROJECT_ID/packages/maven</url>
+  </snapshotRepository>
+</distributionManagement>
+```
+
+In both examples, replace `PROJECT_ID` with your project ID. 
+If you have a private GitLab installation, replace `gitlab.com` with your domain name.
+
+## Configure repository access
+
+If a project is private, credentials will need to be provided for authorization.
+The preferred way to do this, is by using a [personal access tokens][pat].
+You can add a corresponding section to your `settings.xml` file:
+
+
+```xml
+<settings>
+  <servers>
+    <server>
+      <id>gitlab-maven</id>
+      <configuration>
+        <httpHeaders>
+          <property>
+            <name>Private-Token</name>
+            <value>REPLACE_WITH_YOUR_PRIVATE_TOKEN</value>
+          </property>
+        </httpHeaders>
+      </configuration>
+    </server>
+  </servers>
+</settings>
+```  
+
+[pat]: ../profile/personal_access_tokens.md

lib/api/maven_packages.rb

@@ -77,13 +77,13 @@ module API
 
             # Skip handling top level maven-metadata.xml for now
             # Also stop request if version in metadata file differs from one in URL
-            return unless version || version != params[:app_version]
+            return if version.blank? || version != params[:app_version]
           end
 
-          package = Packages::Package.crate(project: user_project)
+          package = Packages::Package.create(project: user_project)
 
-          metadata = ::Packages::MavenMetadatum.crate_by!(
-            project: package.project,
+          metadata = ::Packages::MavenMetadatum.create!(
+            package: package,
             app_group: params[:app_group],
             app_name: params[:app_name],
             app_version: params[:app_version]